Application of aluminum alloys and other light metals and high-strength steel sheets to automobile members has expanded for the purpose of reducing automobile weight, and thereby reducing fuel consumption and other related advantages. However, while light metals such as aluminum alloys have an advantage of high specific strength, their application is limited to special uses because they are far more costly than steel. To further reduce automobile weight, a wider application of low cost, high-strength steel sheets has been highly recommended.
However, when a bending deformation procedure is applied to a work piece of a high-strength steel sheet, because of the high strength thereof, the shape of the work piece thereafter tends to deviate from the shape of the forming jig, and may return to its original shape. The phenomenon of the shape after working of a work piece returning to its original shape is called a “spring back”. When spring back occurs, an envisaged shape is not obtained in the work piece. For this reason, high-strength steel sheets used for conventional automobile bodies have mostly been limited to those having a strength up to 440 MPa.
Although it is preferable to further reduce the weight of a car body by the use of a high-strength steel sheet having a high strength of 490 MPa or more, a high-strength steel sheet showing small spring back and having a good shape fixation property has generally not been available. To enhance the shape fixation property after the working of a high-strength steel sheet having a strength up to 440 MPa or a sheet of a mild steel is generally important for improving the shape accuracy of products such as automobiles and electric home appliances.
Japanese Patent Publication No. H10-72644 describes a cold-rolled austenitic stainless steel sheet having a small amount of spring back (referred to as a dimensional accuracy in the present invention). This publication describes that the convergence of a {200} texture in a plane parallel to the rolled surfaces is 1.5 or more. However, the publication may not include the description related to a technology for reducing the phenomena of the spring back and/or the wall warping of a ferritic steel sheet.
Japanese Patent Publication No. 2001-32050 discloses an invention wherein the reflected X-ray strength ratio of a {100} plane parallel to the sheet surfaces is controlled to 2 or more in the texture at the center of the sheet thickness, and provides certain information regarding the technology for reducing the amount of spring back of a ferritic stainless steel sheet. However, this publication does not refer to the reduction of wall warping and does not include a specification regarding the orientation component group of {100}<011> to {223}<110> and the orientation component {112}<110>, which is an important orientation component for reducing the wall warping.
International Patent Publication No. WO 00/06791 describes a ferritic thin steel sheet whose ratio of reflected X-ray strength of a {100} plane to that of a {111} plane is controlled to 1 or more for the purpose of improving the shape fixation property. However, this publication does not describe the ratios of the X-ray strength in the orientation component group of {100}<011> to {223}<110> to the random X-ray diffraction strength and those in the orientation components of {554}<225>, {111}<112> and {111}<110> to the random X-ray diffraction strength. In addition, there is no disclosure in this International publication of a technology for improving drawability.
Japanese Patent Publication No. 2001-64750 describes a cold-rolled steel sheet, in which, as a technology for reducing the amount of spring back, the reflected X-ray strength ratio of a {100} plane parallel to sheet surfaces is controlled to 3 or more. This publication describes the reflected X-ray strength ratio of a {100} plane on a surface of a steel sheet, and provides that the position of X-ray measurement is different from the position specified in the present invention. In particular, the average X-ray strength ratio in the orientation component group of {100}<011> to {223}<110> is measured at the center of the thickness of a steel sheet. In addition, this publication does not refer to the orientation components of {554}<225>, {111}<112> and {111}<110>, and does not describe the technology for improving drawability.
Further, Japanese Patent Publication No. 2000-297349 describes a hot-rolled steel sheet a steel sheet excellent in a shape fixation property, whereas the absolute value of the in-plane anisotropy of r-value Δr is controlled to 0.2 or less. However, this publication describes improving a shape fixation property by lowering a yield ratio, and it does not include a description regarding the control of a texture aiming at improving a shape fixation property.